The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
There is increasing interest in using electricity generated from renewable resources to supplement or replace the electricity generated by burning fossil fuels. One possible source of such renewable electrical energy is solar energy harvested by photo-voltaic (PV) solar cells.
A PV solar cell is the simplest configuration for converting solar energy into electricity. It is a semiconductor with a p-n junction. Often solar cells are silicon-based and may be single crystal, polycrystalline or amorphous. Compound semiconductor-based PV cells employing compound semiconductor materials such as CdTe and CuInSe2, though less common, also find application. For greater efficiency cells may be constructed in multiple layers, each layer being of a composition adapted to efficiently capture the energy in a specific portion of the solar spectrum, a practice referred to as “spectrum-splitting”.
Most commercial solar cells convert solar energy to electrical energy with an efficiency of about 20% or less. So under full sun, in mid-latitudes corresponding to an incident solar irradiance of about 1000 watts/sq. meter, most PV cells may output only 200 watts/sq. meter or so of electrical energy. Even at the top of the earth's atmosphere the solar irradiance will be less than 1500 watts/sq. meter and in the lower atmosphere values of greater than 1000 are usually attributable to some enhancement due to scattering from reflective surfaces including clouds outside the solar disc. Thus high power PV systems must collect extensive solar power. This may be done by collecting radiation from a wide area and concentrating the energy on a cell of smaller area using mirrors, lenses or other optical devices. More commonly, though, the solar cell area is simply increased by interconnecting and commonly-packaging individual solar cells into larger groupings. Such groupings are often manufacturer-specific and permanent with no opportunity for users to alter or tune the grouping to achieve an output other than that specified by the manufacturer. These groupings are called modules. In turn these modules may be further assembled and connected in series and parallel configurations to generate useful electrical power. Such assemblies are often called arrays. This terminology is in wide use and generally accepted by those skilled in the art. But the electrical characteristics of modules and arrays are not standardized across the industry, and, as noted there is minimal opportunity to alter the cell interconnections within the module. For this reason, the descriptions and discussion herein will use the term PV cell or photo-voltaic cell to connote an individual cell and the terminology, group or grouping of (photo-voltaic) cells to describe any assemblage of electrically interconnected PV cells whether or not the cells are commonly packaged. Thus in industry terminology an exemplary group or grouping could include a number of interconnected individual cells, one or more modules, one or more arrays, or any combination of these.
Typically the voltage delivered by an individual single-layer PV cell is less than a volt while even multilayer cells usually attain little more than two volts. These potentials are less than the voltage required by most electrical or electronic devices, so, to increase the available voltage, a plurality of PV cells is usually grouped by being wired together in series. The current available from such a series-connected PV cell grouping is the same as may be obtained from a single PV cell, only the voltage is increased.
Connecting PV cells in parallel provides more current than a single cell, but at the same voltage. By appropriately connecting the cells of the grouping in both series and parallel as required, the grouping may be adapted to provide a predetermined current at a prescribed potential or voltage. Hence, by appropriately interconnecting PV cells in both series and parallel, a photo-voltaic power unit of suitable current and voltage capability may be assembled.
Because of the nature of PV cells, the solar power which may be harvested by such a cell/module/array is also very sensitive to the requirements of the device (electrical load) to which it is connected. For a given solar irradiance and PV cell temperature, PV cells are capable of developing a voltage which depends on the PV cell composition and the characteristics of the semiconductor p-n junction. But, when connected to an electrical load, the operating voltage delivered to the load will be limited to the voltage demanded by the load device. This may mean that the power delivered by the PV cell may be less than the maximum value that the PV cell is capable of generating. Hence the key to efficient use of solar energy is to operate the PV cell at the voltage where it delivers maximum power to the load. In commercial PV power units, maximum power output is delivered at a particular voltage termed the maximum power point voltage or VMPP.
So any photo-voltaic power source should be matched to a load by substantially matching its VMPP to the operating voltage of the load, and such matching should be maintained despite any variability in PV output. Variability may be result, for example, from changes in solar irradiance, and may be long-term, resulting from the seasonal and diurnal variations in solar irradiance or short-term resulting, for example, from passing clouds.
There is therefore a need for a PV system whose output may be dynamically adjusted to conform to the needs of the load devices powered by the PV power units. Such a system may require a control system and suitable sensors to identify and make appropriate adjustment to the PV system configuration to compensate for both predictable and unpredictable variation in the solar power incident on the PV system.